problem with probabilities

Hi all, I have a problem with probabilities but I can't solve it.The problem is :"I have 3 folders. A folder of them has a letter while the others are empty.The probablity to find the letter in the folder, if we touch the top of the folder is p_j , if there is letter in this folder.We check folder 1 and we did not touch the letter.Which is the possibility the letter to be inside the folder 1 ??"

Re: problem with probabilities

Re: problem with probabilities

In probability, the problem of birthday, or accession to the birthday paradox probability that a number of people chosen at random for a couple of them have the same problem birthday.The is to calculate approximately the probability that a room of people, have at least two the same birthday.